The head of the army, Sir Richard Dannatt, told Jim Naughtie this morning on Radio 4's Today programme that he thought his damning view of the war in Iraq was hardly "newsworthy". Well, that puts him in a minority of one.

Tony Blair and Downing Street certainly understood the news value of the chief of the general staff's interview with the Daily Mail, reportedly debating its consequences into the early hours in a series of conference calls. You can hardly blame them. For the political impact of an intervention like this from Britain's top soldier is huge.

He has said what they never have: that British troops should leave Iraq "soon," because their presence in some areas "exacerbates the security problems" there, rather than reducing them. He called the entire dream of building a liberal democracy in Iraq "naive," and asserts what Blair has always denied: that Iraq is heightening the "difficulties we are facing around the world" - in other words, that Iraq is fuelling Islamist terror. So much for Sir Richard's insistence on the radio that there was not so much as a thin piece of paper between him and the government.

It leaves Tony Blair and his cabinet more isolated than ever, the last true believers in a cause that is almost universally discredited. Opposition from the anti-war movement - even when that movement brought a million or more people onto the streets in 2003 - could be pushed aside. A few commentators or archbishops could similarly be dismissed. But when the head of the military says you're wrong, it becomes ever harder to hold the line. Who could possibly enjoy greater trust among the Middle England voters that Blair cherishes so dearly, than a figure like Sir Richard Dannatt - and speaking to the Daily Mail to boot?

Of course, this will cheer anti-war campaigners no end, a recruit from the highest and least likely level. The Stop the War Coalition welcomed his remarks, urging the government to end an "occupation which is now opposed not only by the majority of the British public but by their own army chief".

But they, and we, should pause a second. Of course it's heartening for those of us who opposed the war from the start to have backing like this. And yet, it's hardly good news. For Sir Richard has just violated a principle central to a democracy: that the military stay well clear of politics. In a democracy, soldiers are meant to be servants of the elected leadership: they follow political decisions, they don't make them.

Of course, Sir Richard will say he was merely speaking up for his personnel and for the army that has been placed in his charge. But he went much further than that, questioning the mission his political masters have embarked on, even elaborating his views on Britain as a Christian country whose values are under threat (with radical Islamism the menace). On matters like this, senior soldiers should keep their mouths shut.

In case they doubt it, anti-war campaigners should imagine the reverse situation. Let's say a new government decided to withdraw from Iraq - only for the chief of staff to tell the Daily Mail he opposed it and believed it would make Britain less safe. Or what if ministers decided not to renew the Trident nuclear system - only for Sir Richard to accuse them of weakening Britain. They would, rightly, be outraged.

This can't be a fair weather principle, important when it goes against us but dispensable when it happens to go our way. No, what Sir Richard said on Iraq was right - but he was wrong to say it.